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Date:   Tue, 5 Nov 2019 09:15:51 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 11/17] arm64: disable function graph tracing with SCS
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On Mon, Nov 04, 2019 at 03:44:39PM -0800, Sami Tolvanen wrote:
> On Mon, Nov 4, 2019 at 9:11 AM Mark Rutland <mark.rutland@arm.com> wrote:
> > Can you please elaborate on _how_ this is incompatible in the commit
> > message?
> >
> > For example, it's not clear to me if you mean that's functionally
> > incompatible, or if you're trying to remove return-altering gadgets.
> >
> > If there's a functional incompatibility, please spell that out a bit
> > more clearly. Likewise if this is about minimizing the set of places
> > that can mess with control-flow outside of usual function conventions.
> 
> Sure, I'll add a better description in v5. In this case, the return
> address is modified in the kernel stack, which means the changes are
> ignored with SCS.

Ok, that makes sense to me. I'd suggest something like:

| The graph tracer hooks returns by modifying frame records on the
| (regular) stack, but with SCS the return address is taken from the
| shadow stack, and the value in the frame record has no effect. As we
| don't currently have a mechanism to determine the corresponding slot
| on the shadow stack (and to pass this through the ftrace
| infrastructure), for now let's disable the graph tracer when SCS is
| enabled.

... as I suspect with some rework of the trampoline and common ftrace
code we'd be able to correctly manipulate the shadow stack for this.
Similarly, if clang gained -fpatchable-funciton-etnry, we'd get that for
free.

Thanks,
Mark.